Fructose

A rule of thumb for determining the D/L isomeric form of an amino acid is the “CORN” rule. The
groups:
COOH, R, NH 2
and H (where R is a variant carbon chain)
are arranged around the chiral center carbon atom. Sighting with the hydrogen atom away from
the viewer, if these groups are arranged clockwise around the carbon atom, then it is the D-form.
If counter-clockwise, it is the L-form.
Properties of enantiomers
Normally, the two enantiomers of a molecule behave identically to each other. For example,
they will migrate with identical Rf in thin layer chromatography and have identical retention
time in HPLC. Their NMR and IR spectra are identical. However, enantiomers behave differently
in the presence of other chiral molecules or objects. For example, enantiomers do not migrate
Nomenclature
• Any non-racemic chiral substance is called scalemic.
• A chiral substance is enantiopure or homochiral when only one of two possible enantiomers
is present.
• A chiral substance is enantioenriched or heterochiral when an excess of one enantiomer is
present but not to the exclusion of the other.
• Enantiomeric excess or ee is a measure for how much of one enantiomer is present compared
to the other. For example, in a sample with 40% ee in R, the remaining 60% is racemic with
30% of R and 30% of S, so that the total amount of R is 70%.
Stereogenic centers
In general, chiral molecules have point chirality at a single stereogenic atom, usually carbon,
which has four different substituents. The two enantiomers of such compounds are said to have
different absolute configurations at this center. This center is thus stereogenic (i.e., a grouping
within a molecular entity that may be considered a focus of stereoisomerism).
Normally when an atom has four different substituents, it is chiral. However in rare cases, two of
the ligands differ from each other by being mirror images of each other. When this happens, the
mirror image of the molecule is identical to the original, and the molecule is achiral. This is called
pseudochirality.
A molecule can have multiple chiral centers without being chiral overall if there is a symmetry
between the two (or more) chiral centers themselves. Such a molecule is called a meso compound.
It is also possible for a molecule to be chiral without having actual point chirality. Common
examples include 1,1’-bi-2-naphthol (BINOL) and 1,3-dichloro-allene, which have axial chirality,
(E)-cyclooctene, which has planar chirality, and certain calixarenes and fullerenes, which have
inherent chirality.
It is important to keep in mind that molecules have considerable flexibility and thus, depending
on the medium, may adopt a variety of different conformations. These various conformations are
themselves almost always chiral. When assessing chirality, a time-averaged structure is considered
and for routine compounds, one should refer to the most symmetric possible conformation.
When the optical rotation for an enantiomer is too low for practical measurement, it is said to
exhibit cryptochirality.
Even isotopic differences must be considered when examining chirality. Replacing one of the
two 1H atoms at the CH2 position of benzyl alcohol with a deuterium (²H) makes that carbon a
stereocenter. The resulting benzyl-α-d alcohol exists as two distinct enantiomers, which can be
assigned by the usual stereochemical naming conventions. The S enantiomer has [α]D = +0.715°.
identically on chiral chromatographic media, such as quartz or standard media that have been
chirally modified. The NMR spectra of enantiomers are affected differently by single-enantiomer
chiral additives such as Eufod.
Chiral compounds rotate plane polarized light. Each enantiomer will rotate the light in a different
sense, clockwise or counterclockwise. Molecules that do this are said to be optically active.
Chacteristically, different enantiomers of chiral compounds often taste and smell differently and
have different effects as drugs – see below. These effects reflect the chirality inherent in biological
systems.
One chiral ‘object’ that interacts differently with the two enantiomers of a chiral compound is
circularly polarised light: An enantiomer will absorb left- and right-circularly polarised light to
differing degrees. This is the basis of circular dichroism (CD) spectroscopy. Usually the difference
in absorptivity is relatively small (parts per thousand). CD spectroscopy is a powerful analytical
technique for investigating the secondary structure of proteins and for determining the absolute
configurations of chiral compounds, in particular, transition metal complexes. CD spectroscopy is
replacing polarimetry as a method for characterising chiral compounds, although the latter is still
popular with sugar chemists.
In biology
Many biologically active molecules are chiral, including the naturally occurring amino acids (the
building blocks of proteins), and sugars. In biological systems, most of these compounds are of
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